The present invention relates to refrigeration systems and more particularly relates to expansion valves for use in refrigeration systems. The present invention is described herein in detail with respect to a conventional refrigeration system. However, those of ordinary skill in the art to which the present invention pertains will readily recognize the broader applicability of the present invention. For example, the present invention may find application in a heat pump system, an air conditioning system, or other such system.
Conventional refrigeration systems utilize a recirculating refrigerant for removing heat from the low temperature side of the refrigeration system and for discharging heat at the high temperature side of the refrigeration system. The work input necessary to operate the refrigeration system is provided by a motor driven compressor which receives low pressure gaseous refrigerant and compresses it to a high pressure. This high pressure gaseous refrigerant is supplied to a condenser where heat is removed from the gaseous refrigerant to condense it to a liquid. This liquid refrigerant is then supplied through an expansion valve to an evaporator wherein heat is transferred from a heat transfer fluid used to cool a load, such as to cool a room, to the liquid refrigerant to evaporate the liquid refrigerant. This gaseous refrigerant from the evaporator is then returned to the compressor for recirculation through the refrigeration system.
The amount of heat absorbed by the refrigerant liquid in the evaporator includes the heat of vaporization of the liquid refrigerant; that is, the amount of heat which must be absorbed by a liquid at a given temperature to convert it to a gas at the same temperature. In addition, the gaseous refrigerant in the evaporator may absorb additional heat which raises its temperature above the temperature of vaporization. Such gaseous refrigerant is said to be superheated, and the amount by which the temperature of the gaseous refrigerant is raised above the vaporization temperature is expressed in degrees of superheat. Ideally, the expansion valve should admit an amount of refrigerant that can be evaporated and just slightly superheated in the evaporator. That is, the evaporator should be "wetted" with refrigerant along approximately its entire length to provide good heat transfer and maximum refrigeration system efficiency. Conventionally, a portion of the evaporator always has been operated dry to prevent passage of liquid refrigerant to the suction side of the compressor since liquid entering the suction side of the compressor may damage the compressor.
The expansion valve and its control system play important parts in overall refrigeration system operation and in overall refrigeration system efficiency. It is highly desirable to provide a refrigeration system with an expansion valve which can precisely control refrigerant flow through the valve, which can provide other functions such as a shutoff function, and whose operating position can be simply, reliably, and accurately determined during operation of the valve. For example, superheat and refrigeration system evaporator wetting characteristics may be optimized to optimize refrigeration system operating efficiency with an expansion valve which can precisely control refrigerant flow from the condenser to the evaporator of the refrigeration system. Also, if the expansion valve can provide a shutoff valve function in addition to controlling refrigerant flow this eliminates the need for using a liquid line solenoid valve as part of the refrigeration system. Further, if operating position of the expansion valve is always known then a control system for the refrigeration system may be programmed to more effectively and efficiently operate the refrigeration system by carrying out certain control actions based on expansion valve position.
However, known refrigerant expansion valves are not particularly well suited to provide the functions described above. For example, thermostatic expansion valves, which are probably the most prevalent type of expansion valve used in refrigeration systems, are not particularly suitable for precisely controlling refrigerant flow through the valve to optimize refrigeration system operating efficiency by precisely controlling superheat and evaporator wetting characteristics. When thermostatic expansion valves are used to control superheat and evaporator wetting characteristics they generally suffer, among other shortcomings, a slow response time, a wide regulating range, and a narrow load range. In addition, thermostatic expansion valves are not well suited to provide other functions such as a shutoff valve function. Further, it is difficult to accurately determine the operating position of thermostatic expansion valves during operation of the valves.
Electrically controlled expansion valves with analog control systems also are known. The analog control system opens and closes the valve in response to refrigeration system operating conditions sensed by sensors providing input signals to the analog control system which are a function of the sensed refrigeration system operating conditions. Examples of such electrically controlled expansion valves with analog control systems are disclosed in U.S. Pat. Nos. 4,362,027 to Barbier, 4,067,203 to Behr, 3,967,781 to Kunz, and 3,577,743 to Long. In general, such electrically controlled expansion valves should be able to control refrigeration systems more precisely than thermostatic expansion valves because of the theoretically faster response time of the electrically controlled expansion valves. However, while such electrically controlled expansion valves may be an improvement over thermostatic controlled expansion valves, these electrically controlled expansion valves are not ideal in operation. In general, they are not particularly suitable for maintaining desired levels of refrigerant superheat and/or evaporator wetting characteristics to optrmize refrigeration system efficiency without undesirable hunting of the expansion valve or flooding of the refrigeration system compressor. Also, some of these electrically controlled expansion valves are not capable of providing other functions such as a shutoff valve function. Further, it is not particularly easy to reliably and accurately determine the operating position of such electrically controlled expansion valves during their operation.